Polyamide (i.e. nylon) resin reinforced with mineral filler and small amounts, e.g. about 0.5%, of silane coupling agent can be molded into parts which exhibit improved stiffness when compared with the properties of the unmodified nylon resin. Suitable silanes are described in the prior art, for example, U.S. Pat. No. 3,290,165 discloses the use of gamma aminopropyltriethoxysilane as a coupling agent. Examples of mineral filler nylon resin are provided in U.S. Pat. No. 3,419,517. However, the falling-weight impact resistance of mineral filled nylon resin is inferior to that of the parent nylon resin which does not contain filler. To overcome this deficiency, the impact resistance of filled nylon resins has been improved by the use of a blend of nylon-6 and nylon-66 (see for example, U.S. Pat. No. 3,846,367) or by the use of toughening polymers with low modulus (see for example, U.S. Pat. No. 4,174,358). However, molded parts derived therefrom showed an undesirable sacrifice of stiffness as measured by flexural modulus when subjected to humid environments. It is an object of this invention to improve the impact resistance of mineral filled nylon resin without sacrificing the modulus of a specified polyamide matrix.
Ordinarily the dispersion of a high concentration of mineral filler into a polyamide matrix requires the use of heavy compounding equipment, such as a continuous mixer based on the principle of the Banbury mill with counter-rotating blades, or a twin screw extruder with co-rotating, intermeshing screws. It would be desirable to use a simple single screw extruder to attain a uniform dispersion of filler in polyamide, in order to reduce the capital investment of this processing step. With a single screw extruder, the capital investment per unit weight of product per year is about one-half that of a twin screw extruder. Therefore it is a further objective of this invention to provide a composition that will yield a uniform dispersion by compounding on a simple single screw extruder.
A physica test commonly employed to judge the degree of a given dispersion of filler in polyamide is the falling-weight impact test (ASTM D-3029) on specimens molded from the filled composition. If agglomerates of filler are present. they will act as stress risers in the molded specimen, and the impact resistance will be lowered. On the other hand, if the dispersion of filler is uniform on a microscopic level, this fact will be demonstrated by material with essentially equivalent impact resistance, whether the given composition is derived from a single screw extruder or from heavy compounding equipment.
The use of sulfonamides as plasticizers for nylon is disclosed in U.S. Pat. Nos. 2,214,405 and 2,757,156. Neither patent refers to mineral filled nylon. It is well known that when a sufficient concentration of sulfonamide is present to act as a plasticizer for the polyamide, both the tensile strength and modulus decline. Thus there is a serious compromise between improved toughness and loss of rigidity in the molded part.